1. Field of the Invention
This invention relates to an image fiber of small diameter for providing a high image quality, which is chiefly applied to endoscopes for medicine, and a method of fabricating the image fiber.
2. Description of Related Art
In general, for medical endoscopes using image fibers of this type, an arrangement, such as that shown in FIG. 1, has been well known from the past. In an endoscope apparatus 21, an object T illuminated by an illumination optical system, not shown, is imaged through an objective unit 22 at an entrance end 23a of an image fiber 23 constructed with the bundle of a plurality of optical fibers. An observation image T' thus formed is transmitted through the image fiber 23 and is observed through an eyepiece 34 at an exit end 23b.
Such image fibers are well known as flexible image fibers in which individual optical fibers constituting the image fiber are consolidated at their entrance and exit ends and are separated in their middle portions, and as conduit image fibers in which individual cores have a common cladding integrally constructed.
FIG. 2 shows a schematic sectional structure of the conduit image fiber. A conduit image fiber 25 has a plurality of cores 27 which are arrayed within a common cladding 26 in mutually spaced relation.
In the endoscope apparatus 21 using the image fiber of this type, its outside diameter has been more and more reduced in recent years. More recently, research and development have been made to design endoscopes 1 mm or less in diameter so that, for example, the interiors of blood vessels can be observed. In keeping with this, the image fiber 23 applied to the endoscope apparatus 21 has also become very thin.
However, individual optical fibers constituting the image fiber 23 of the endoscope apparatus 21 are each as small as several hundred micrometers in outside diameter and include cores each having a diameter of several micrometers to form two to three thousand pixels. Thus, satisfactory image quality is not necessarily brought to the observation image T'.
In the image fiber which will be described below, a core radius a, cladding thickness t, and core-to-core spacing P are defined by the notation shown in FIG. 3.
Of the above-mentioned image fibers, the flexible image fiber maintains its flexibility even if its diameter is increased. When the diameters of individual optical fibers are reduced, however, the image fiber is liable to break. Moreover, because the thickness of the cladding becomes smaller, light on the long-wavelength side leaks from the cladding and is colored blue, which is not desirable. Consequently, there is the disadvantage that the mutual spacing of the optical fibers cannot be reduced to about 10 .mu.m or less.
The conduit image fiber, on the other hand, is often used in an endoscope of very small diameter. Although it is possible to reduce the mutual spacing of the optical fibers because the cores have a common cladding, if the outside diameter of the image fiber becomes greater than nearly 500 gm, its flexibility will be conspicuously lacking. As such, there is the problem that it is impossible to increase the diameter of the image fiber while maintaining the flexibility.
In the conduit image fiber 25 constructed as mentioned above, the common cladding 26 extends continuously along the optical axis, and hence the mutual spacing of the cores 27 can be made narrower than that of the flexible image fiber. However, it is known that part of light transmitted through the cores 27 leaks out through the interface between the cladding 26 and each of the cores 27 into the cladding 26, and the phenomenon that the component of the leaked light enters another core 27, namely what is called cross talk, is liable to produce. This cross talk phenomenon becomes particularly pronounced and forms a great cause for degrading the image quality when the thickness of the cladding 26 is no more than several times the wavelength of light, for example, the mutual spacing of the cores 27 is reduced to 10 .mu.m or less.
In recent years, it is desired that even the image fiber 23 of very small diameter provides a high density of pixels and bright images, that is, is high In the core area ratio of the cores 27 (pixel density, namely the ratio of cores to an image fiber per unit sectional area).
For this purpose, it is required that the diameters of the cores 27 are reduced and the thicknesses of the cladding 26 between the cores 27 are made small to increase the core area ratio.
If, however, each thickness of the cladding 26 between the cores 27 is reduced to several times or less the wavelength of light, cross talk will be liable to occur because of the mode coupling between light through the optical fibers, thus causing the quality of transmitted image to degrade remarkably. In order to prevent the degradation of image quality, it is necessary to increase the thickness of the cladding 26. This, however, decreases the core area ratio of the cores 27. Consequently, the defect is encountered that a high density of pixels as well as bright images cannot be secured.
Thus, there is the discrepancy that in order to reduce cross talk, the thickness of the cladding 26 must be increased, while for improving the core area ratio, the cladding 26 must be made thinner.
In the image fiber of this type, it is generally known that its constituent optical fibers in which light propagated through the cores has different propagation constants .beta. are less in cross talk than those in which the light has the same propagation constant .beta.. In order to vary the propagation constants .beta. of the light propagated through the cores, it is only necessary to vary the values of normalized frequencies V. It is seen that the value of the normalized frequency V is dependent on the core radius a as expressed by ##EQU1## where k=2.pi./.lambda. (.lambda. is the wavelength of light propagated through each optical fiber) and n.sub.1 and n.sub.2 are refractive indices of each core and the cladding, respectively.
A parameter generally referred to as the numerical aperture (NA) of a fiber is given by ##EQU2##
As one of provisions made for cross talk in the image fiber by devoting attention to the above respect, what is called a random image fiber, such as that disclosed in Japanese Patent Publication No. Hei 3-81126 or Hei 3-77962, has been known in which a plurality of optical fiber elements having cores of different diameters are bundled into an image fiber. The conventional random image fiber, however, has an unfavorable problem that since the optical fiber elements used are few in kind and are randomly bundled, cases not unfrequently occur in which fiber elements having cores of identical diameters are adjacent one another, and the image quality is degraded by cross talk produced between the fiber elements of identical core diameters.
In this image fiber, since its individual cores are also arranged in a random array, the resolving power of a transmitted image may be partially uneven and a straight edge may appear to curve. This makes the observation of the transmitted image difficult.
An image fiber whose individual cores are random in diameter and are arranged in a hexagonal close-packed array is disclosed, for example, in Japanese Patent Preliminary Publication No. Hei 4-214042. Even with this image fiber, optical fibers which are different in core-to-cladding ratio (the diametric ratio between a core and an optical fiber element) are randomly arranged, and hence there is the problem that, for example, when an object with even brightness is observed, unevenness in brightness is caused to the observation image itself.